This invention relates to a system and method for fastening rivets and/or using process indicators to communicate to operators the stage of each rivet during a rivet setting cycle. In particular, the invention relates to a system and method that relies on a micro-adjustable switching mechanism that is used as part of a feedback control system to achieve rivet setting tolerances by measuring in real-time or near-real-time the rivet's driven head (sometimes called the upset head or shop head) height while the control system also controls rivet gun operation and communicates the rivet driving-cycle stage to the rivet setting operator(s).
Riveting produces the strongest practical means of fastening airplane skins and substructure together. Although the cost of installing one rivet is small, installing the great number of rivets used in airplane manufacture represents a large percentage of the total cost of any airplane.
It should be first noted that the term “tolerance” is used broadly throughout this disclosure. Conventionally, the term tolerance signifies a plus or minus range of acceptance on a bell-shaped-curve distribution of samples with preferably the peak of the bell-shaped curve representing the optimum bounded by narrow bandwidth indicating very small standard deviations. The curve is used to quantitatively characterize defects. In this disclosure, the term tolerance also sometimes refers to a specific value representing the optimum peak of the bell-curve (or very near peak, i.e., extremely tight tolerance). For example, “It is often difficult to consistently set rivets to meet tolerances but it is extremely difficult to consistently set rivets to an optimal tolerance.”
Although this invention may be applied to special types of rivets, for purposes of clarity, this disclosure uses as an example conventional solid-shank rivets that comprise a manufactured head, a shank and a driven head. The driven head is formed by upsetting the rivet shank with a rivet gun while backing the shank with a bucking bar. The shank actually expands slightly while being driven so the rivet fits tightly in the drilled hole.
Where there is easy access to both sides of the work, the rivet-gun operator can sometimes simultaneously drive the rivet and back the rivet with a bucking bar; however in most cases both a rivet-gun operator and a bucking-bar operator or bucker must work together to drive solid-shank rivets. The conventional procedure for driving rivets is as follows: (1) the rivet gun operator adjusts the air regulator which controls the pressure or hitting force of the pneumatic rivet gun; next (2) the rivet gun operator inserts the rivet into the drilled hole, places the rivet set tool face against the rivet and waits for the bucker; next (3) the bucker holds the bucking bar on the protruding shank-end of the rivet; next (4) the rivet gun operator should “feel” the pressure being applied by the bucker through the rivet; and finally (5) the rivet-gun operator will start the rivet gun by pulling the trigger to release a short burst of rivet-gun blows and then stop the rivet gun when the rivet has been driven or set to be within a desired range of manufacturing specifications or tolerances.
Throughout the rivet setting process, both operators must hold their tools perpendicular or orthogonal to the work so the rivet is driven axially. The entire rivet setting process requires both skill and experience since the rivet-gun operator must determine rivet gun burst-length or blows needed according to variables such as bucking resistance being applied, the rivet size being driven, the rivet gun pressure setting and the mass of the rivet gun and bucking bars. These variables must be judged by the rivet-gun operator to time the length of the rivet driving stage needed to achieve rivet setting tolerances.
Further, to communicate with each other, the rivet-gun operator and bucker conventionally use a tapping code to enable to bucker to communicate with the rivet-gun operator: one-tap on the rivet by the bucker means start or resume driving the rivet (resuming is often necessary when the rivet has been under-driven and has not reached tolerance); two-taps on the rivet by the bucker means the finished or set rivet was within satisfactory tolerance; three-taps on the rivet by the bucker means the rivet was improperly set and must be removed (this typically occurs when the rivet has been over-driven and can not be modified to achieve tolerance). Where verbal communication is possible, the rivet-gun operator typically announces “ready” when he is ready to begin riveting and waits for the bucker to likewise announce “ready” when he is ready to begin bucking and follows with a “good”, “drive more” or “not good” verbal report of the completed set rivet.
To achieve design strength, the driven head of a rivet must fall within an acceptable tolerance range; to inspect rivets, the bucker sometimes uses a gauge to measure the driven head-height or driven head-width after the rivet has been set. Often, however, to save time, the bucker only visually inspects the driven head to determine if it meets required tolerances. If the rivet has been under-driven leaving the head height too high, additional driving is needed (although due to work hardening of the rivet material, rivet holding strength for rivets driven in repeated driving stages is often reduced). Over-driven rivets require removal, which is a time consuming process that can often damage the work and sometimes requires using an oversized replacement rivet having a different setting tolerance. Over-driven rivets often blemish or bend the work, sometimes causing costly rework or irreparable damage.
The background art is characterized by U.S. Pat. Nos. 1,803,965; 2,354,914; 3,478,567; 3,559,269; 3,574,918; 3,933,025; 4,218,911; 4,566,182; 5,398,537; 5,953,952; 6,011,482; 6,088,897; 6,357,101; 6,363,768; 6,823,709; and 7,331,205; the disclosures of which patents are incorporated by reference as if fully set forth herein.
Although the conventional method of driving rivets described above has been effective for many years, there are some inventions that attempt to improve the process. As an example, U.S. Pat. No. 5,953,952 by Strickland proposes a micro-adjustable bucking bar anvil to set the distance between the anvil face and the spindle's feet base to match desired driven head height of a set rivet. Strickland further proposes that the spindle's feet help the bucker maintain axial alignment of the tool relative to the rivet shank and orthogonal alignment of the bucking tool relative to the work surface. Finally, Strickland proposes use of a compression spring working between the bucking tool and the work to hold the work sheathing pieces together while riveting.
In another example, U.S. Pat. No. 6,363,768 by Earls and Bland simplifies the Strickland design by proposing a precision bucking bar having a recessed anvil face with the equivalent of non-adjustable spindle's feet formed by their nearest reference as “sidewalls.” This invention requires that the bucker choose a bucking bar having “sidewalls” the same height as the desired driven head of the set rivet.
In both examples above, however, the bucker must visually identify when the driven head is finished (identified when the spindle's feet or equivalent make contact with the work) and then the bucker must immediately signal the rivet-gun operator to stop the rivet gun. This communication from the bucker to the rivet gun operator to “stop riveting” is difficult to achieve because no adequate means to affect this communication, during the loud riveting process, is proposed. Furthermore, due to reaction times of both operators and the fact that a rivet gun typically hammers at rates exceeding 20 Hertz, it is unlikely that these methods could achieve consistent desired rivet setting tolerance control. Most importantly, if the rivet gun were not immediately stopped at the moment the bucker visually identified rivet set completion, the additional impacting forces from the rivet gun would be imparted through the rivet to the anvil face and from the set-rivet through the work to the spindle's feet resulting in the spindle's feet causing damage to the work. Damage to the work could include bending, marring, crushing and/or scratching. In addition to reduced strength from airframe damage or substructure damage, damage to the anodized work surfaces could also result in premature corrosion. It is important to note in both these inventions that rivet head achieves the desired set tolerance only when the spindle's feet touch the work and that this contact requires visual identification by the bucker. Due to the vibratory nature of riveting, this would be difficult to reliably observe. Furthermore, since the spindle's feet do not rest against the work until the rivet is set, the spindle's feet are a poor tool alignment aid.
In yet another example, U.S. Pat. No. 6,011,482 by Banks et al. requires massive rail-mounted riveting equipment operating on each side of the work components being fastened together; the equipment requires costly computer numerically controlled (CNC) position control machines and extensive capital costs for the rivet driving machinery. The reference states near line 60 that the manual “process results in rivets that were unevenly deformed, poorly seated” and near line 65 that “unfortunately, the manual process is dangerous, time consuming, expensive and often leads to extensive rework.” Also, the Banks invention only “determines the acceptability of the rivet within a component” and does not control the rivet driving process to achieve an optimal set of a driven rivet head. In another example, U.S. Pat. No. 6,357,101 by Sarh et al. similarly requires massive rail mounted riveting equipment that is beyond the scope of most common manual rivet installation applications.
In another example, in U.S. Pat. No. 7,331,205 by Chitty et al., a rivet gun technique is proposed that measures set tool strain and rivet gun pressure in a rivet gun to set blind rivets. In other words, continuous analog sensor measurement of a hydraulic rivet gun pressures are used to access the driven head throughout its forming process; this assessment is coupled with controlling rivet gun impact force and measuring driving time are used to directly control set rivet material strength and thus control rivet holding strength. While the Chitty et al. invention is used for setting blind rivets, the reference does not teach use of measured deflections of the rivet head over time and assessment of the number of impacts needed to determine optimal rivet gun pressure settings while also still maintaining settings within ranges acceptable for manual operation.
None of the references teach or suggest the invention disclosed herein. What is needed is a rivet fastener system that overcomes the disadvantages of the background art. To overcome the disadvantages of the background art, a rivet fastening system is disclosed herein.